We propose to continue our studies of the responses of bacteria to UV induced DNA damage. This entails purifying bacteriophage T4 endonuclease V, an enzyme specific for pyrimidine dimers, and using this enzyme to monitor DNA repair. Having completed studies of post-replication repair in tsl mutants of Escherichia coli K-12, we now plan to examine repair in tif-l mutants. Like tsl mutations, tif-l affects the amount of recA protein produced by E. coli cells. However, unlike the tsl mutations, tif-l lies within the recA gene and alters the structure of the recA protein. Since recA is one of the genes determining the capacity for post-replication repair we plan to ascertain whether the tif-l mutation affects post-replication repair and whether transdimer synthesis can be detected in UV-irradiated tif-l cells in which SOS functions have been thermally induced. In addition, we hope to determine the biochemical basis for the enhanced resistance to UV of recA cells starved for required amino acids.